1. Field of the Invention
The present invention relates to a process for preparing dialkyl carbonates. More particularly, the present invention relates to a process for preparing dialkyl carbonates via the reaction of carbon monoxide, alkanol and an oxygen-containing gas in the presence of an ionic halogen free catalyst. The present invention also relates to a composition useful for the preparation of dialkyl carbonates, the composition comprising an ionic halogen free catalyst, carbon monoxide, an alkanol and an oxygen-containing gas.
2. Description of Related Art
Industrially, dimethyl carbonate (DMC) is used in the production of polycarbonates and has the potential to be used as an environmentally friendly fluid for numerous solvent related applications and conceivably even as a fuel oxygenate (e.g., methyl tertiary butyl ether replacement).
Historically, DMC was prepared from the highly toxic intermediate phosgene, COCl2. Currently, it is prepared via oxidative carbonylation of methanol using an insoluble copper(I) chloride catalyst. This method is based on copper(I) chloride as the catalyst and demonstrated in EP 534,545 B1 and EP 460,732 A1. The overall copper catalyzed reaction of carbon monoxide (CO), methanol (CH3OH) and oxygen (O2) is shown in equation (1) below:CO+2CH3OH+½O2→(CH3O)2CO+H2O  (1)
The copper(I) chloride catalyst is very insoluble in this system and, thus, is a limiting component in the catalytic cycle. Hydrochloric acid is also added as a component in this oxidative carbonylation system. This was done to prevent the oxidation of Cu(I) to Cu(II) in the presence of oxygen and water since Cu(I) is believed to be the active species in this system. This copper chloride catalyst-based oxidative carbonylation system, which is run between 120 and 160° C., is extremely corrosive and requires costly components (e.g., glass lined reactors). Failure in the glass lining could lead to rupture or explosion. Two other processes for the production of DMC are disclosed in U.S. Pat. Nos. 6,010,976 and 5,498,744. U.S. Pat. No. 6,010,976 discloses an organotin catalytic reaction of urea with methanol to first form the carbamate, which is further reacted to form DMC, ammonia and carbon dioxide. U.S. Pat. No. 5,498,744 discloses a process that reacts methylnitrite with carbon monoxide over a palladium catalyst to form DMC and (NO)x, which is toxic.
DMC, due to its low toxicity and low atmospheric reactivity, has tremendous growth potential as a possible replacement for methyl tertiary butyl ether (MTBE), as a fluorocarbon solvent replacement in the electronics industry and as an environmentally-friendly solvent for use in the production of polycarbonates. The problems with MTBE, fluorocarbons, and phosgene are widely publicized. The growth of DMC use has been, in part, limited by the difficulties in commercial production. An efficient and environmentally-friendly method for the large-scale production of DMC would be desirable, especially a process that eliminates the need for a chloride-based catalyst which causes corrosion of the reaction vessel and impurities in the resultant product.
There exits a need for a non-corrosive process for the production of dialkyl carbonates, particularly DMC, prepared from carbon monoxide, an oxygen-containing gas and an alcohol. There also exits a need for a soluble catalyst for use in the non-corrosive process for the production of dialkyl carbonates.